writing functions in assembler

(assuming NASM x86)

Use call in order to call the function and ret to return from the function.

What occurs when you type call is that the address of the next instruction is pushed into the stack. When ret is hit, it will pop that address off the stack and jmp to it.

func:
    xor eax, eax
    mov eax, 10
    add eax, 5
    ret ;// essentially identical to: pop [register] -> jmp [register]


_start:
    call func
    mov ebx, eax ;// Address of this instruction is pushed onto the stack
    ;// ebx is now 15

Calling convention dictates that the EAX register should contain the return value. Also note that the __cdecl calling convention takes parameters on the stack. Take a look at the examples in the afore-linked page. The NASM function will set up its stack frame and take parameters from the stack in order to use in the function. The value is stored in EAX.